Too risky

They were went after hard by law enforcement and if they couldn't get them on anything illegal then the credit card companies went after them. It came to the point that it just wasn't worth it anymore. The risk involved became too high. And it wasn't just risky for the people running the sites but to those who joined as well. I'm sure a lot of potential customers were scared away by the possibility of the police knocking down their door. If it wasn't for that, they would still be around today bigger and more popular than ever.